1. Field of the Invention
The present invention relates to a method for manufacturing an ink jet head, and an ink jet head manufactured by such method.
2. Related Background Art
An ink jet head is arranged to discharge ink from its nozzles as fine droplets for recording characters, images, and others. It has outstanding advantages as means for outputting images having high precision, as well as for printing at high speeds. Particularly, the method that uses pressure exerted by bubbles (air bubbles) created by electrothermal transducing elements (hereinafter referred to as heaters) or the like, that is, the so-called thermal ink jet recording method (disclosed in U.S. Pat. No. 4,723,129, is characterized in that such method enables an apparatus to be manufactured compactly, and that it makes it easier for the apparatus to record images in high density, among other advantages. FIG. 14 illustrates a thermal ink jet head described above as one example. FIG. 14 is a perspective view which shows the so-called side shooter type thermal ink jet head. FIG. 15 is a perspective view which shows the heater board that constitutes the head represented in FIG. 14.
The ink jet head shown in FIG. 14 is structured by bonding a nozzle plate member 102 having a plurality of orifices 101 arranged therein together with a substrate 103. On the substrate 103, an ink supply inlet 104 is opened as shown in FIG. 15. On the surface of the substrate 103, which is bonded to the nozzle plate member 102, a plurality of heaters 105 are arranged corresponding to the positions of the orifices 101.
Also, FIG. 16 is a cross-sectional view taken along line 16--16 in FIG. 14. As shown in FIG. 16, there are provided between the substrate 103, and the nozzle plate member 102, a liquid chamber 106 conductively arranged from the ink supply inlet 104 to the orifice 101 arranged above the heater 105, and a nozzle 107. Ink is supplied to the nozzle 107 from the ink supply inlet 104 through the liquid chamber 106. Then, ink is discharged from the orifice 101 by means of the pressure exerted by bubbles created on the heater 105.
The characteristic structure of the ink jet head described above is such that the space needed for the liquid chamber and the nozzle is formed by bonding the substrate 103 and the nozzle plate member 102 together.
This head can be structured by the steps of manufacture shown in FIGS. 17A to 17G. Hereunder, with reference thereto, the description will be made of a method for manufacturing an ink jet head described above.
A substrate 103 having the ink supply inlets 104 and heaters 105 provided in advance is prepared (see FIG. 17A). Then, a photoreactive positive type resist material 107, such as a dry-filmed ODUR (product name--manufactured by Tokyo Ohka Kabushiki Kaisha), is laminated thereon (see FIG. 17B). A molding member 109, which provides nozzles and a liquid chamber, is formed on the substrate 103 by means of photo-lithographic process (see FIG. 17C). The surface configuration of this molding member 109 is shown in FIG. 18. In FIG. 18, the portions designated by reference marks B and C are those where the nozzles and the liquid chamber are formed, respectively.
Then, by dissolving the following mixture into a solvent of xylene/cyclohexane=8/2 by 50 wt %, a resin material is obtained; this resin material is spin coated on the substrate 103 and the molding member 109 and hardened by use of light or heat, thus forming a nozzle plate member 102 (see FIG. 17D):
Nozzle plate material:
______________________________________ Epicoat 1002 (product name - Yuka Shell Epoxy KK) 100 parts Epolite 3002 (product name - Kyouei Kabushiki Kaisha) 20 parts Irgacure 261 (product name - CIBA GEIGY) 3 parts ______________________________________
After this process, an oxygen-proof photohardening plasma material 110 is coated to form a thin film on the nozzle plate member 102, and then, removed sections 111 are formed by photolithographic process each in the shape of an orifice in a given position: here, the position facing each of the heaters (see FIG. 17E). Thus orifices 101 are formed on the nozzle plate member 102 by means of plasma irradiation (see FIG. 17F). The molding material 109 is dissolved and removed through the orifices and the ink supply inlets for the formation of the nozzles 107 and the liquid chamber 106 (see FIG. 17G).
The performance of ink discharge from the ink jet head produced by the method of manufacture described above depends greatly on the gap between the heater surface and the orifice formation surface. However, the structure being such that the nozzle plate member is formed by coating the resin material, it is easy to control the gap between the heater surface and the orifice formation surface. This gap exerts a serious influence on the ink discharge characteristics when heads are manufactured. The structure thus arranged also contributes to manufacturing them at lower costs. Further, it is possible to provide small droplets of less than 10 pl. Such small droplets are needed particularly for obtaining images having high precision. Moreover, since the orifices are formed by means of a photolithographic process, it is easy to position the heaters and orifices, among other features. A method for manufacturing a nozzle plate member by coating a resin material on a substrate having such a molding member on it is, hereinafter, referred to as a "resin plate injection molding method".
However, if a nozzle plate member as extremely thin as 100 .mu.m or less should be formed by means of the manufacturing process shown in FIG. 3 in view of the fact that the narrower the gap between the heater surface and the orifice formation surface, the better the ink discharge characteristics, the coating condition of resin material on the nozzle plate member may sometimes become uneven in the vicinity of the corners of the extruded molding member on the substrate.
Now, with reference to FIG. 18 and FIG. 19, the description will be made of the problems to be encountered if such unevenness occurs. FIG. 19 is a cross-sectional view which shows the head portion when an extremely thin nozzle plate member is formed by means of the resin plate injection molding method.
In other words, a problem arises at a portion indicated by a reference mark E in FIG. 19, which corresponds to the portion D in FIG. 18. The thickness of the resin material coated on the substrate becomes locally thinner in the vicinity of the extruded corners of the molding member that produces the liquid chamber on the substrate. As a result, stress is concentrated on this thinner portion to create a crack 112 on the nozzle plate member. In a serious case, the liquid chamber is caused to sink in, resulting in an unfavorably reduced yield when ink jet heads are produced.
In order to avoid this drawback, it should be arranged to make the difference between the film thickness H of the nozzle and liquid chamber portion, and the film thickness h of the portions other than such portion as small as possible: preferably, the thicknesses H should be approximately equal to the thickness h, that is, the surface of the nozzle plate member should be made substantially flat. However, it is difficult to make any improvement in this respect just by devising some method for coating a resin material. Here, also, the process becomes complicated if coating should be repeated several times to obtain a flat surface, which inevitably brings about increased costs of ink jet head manufacture. Further, in order to improve the resin coating condition at the extruded corners of the molding member with respect to the substrate, it may be conceivable to coat the nozzle plate member in a sufficient thickness taking the thickness of such molding member into account. In this case, however, the resultant gap between the heater surface and the orifice formation surface becomes greater, thus making it difficult to design nozzles that can obtain specific discharge characteristics.